GK - Not quite true.
There is a vulnerability spec for the F-35 (as for all US fighters since VN, I believe, hence the live-fire testing) that probably includes something to the effect that no single projectile or fragment (from an AAM or SAM warhead) above x grams and y meters/sec shall cause loss of aircraft.
The lift fan is light (no high-speed birdstrike requirement) and relatively exposed. The risk identified by DOT&E is that the fan is damaged and nobody knows it, and consequently fails just when the flight/propulsion control system thinks that it is starting to produce thrust, and as the aft nozzle is moving downwards. Result: large pitch moment that there is nothing to counteract.
How important this is in the big survivability picture, I don't know. But the spec is there and presumably this is a risk of a miss, because that's what DOT&E's terms of rerference are.